The invention relates to a waste gas cleaning system for the disposal of environmentally harmful and/or toxic gases or vapors having a reaction chamber that is connected with a plasma source in which a plasma is formed by coupled excitation energy, where the reaction chamber and/or the plasma source has at least one inlet for the feed of gases or vapors and the reaction chamber has an outlet for the gases or vapors treated in the plasma source and/or in the reaction chamber.
A variety of methods have been disclosed for cleaning environmentally harmful and/or toxic gases or vapors, such as for example waste gases coming from semiconductor manufacturing processes such as CVD, LP-CVD, plasma-CVD, plasma etching or similar processes. In the great majority of cases, methods in which the gases or vapors are burned and/or thermally decomposed are used. The thermally treated gases or vapors are then passed through a scrubber in which the solid and/or soluble reaction products are scrubbed out of the waste gases by means of a sorption medium.
Such a device for cleaning waste gases has been disclosed in WO 96/23173. This device contains a combustion chamber in which is arranged a burner and to which are fed burnable gas, oxygen or air and the process waste gas to be decomposed. Above the combustion chamber is located a scrubbing chamber with a spray nozzle for spraying the sorption medium. There the combustion chamber is located within an outer pipe and is limited by an inner pipe, the outer pipe also surrounding the scrubbing chamber that is located above the combustion chamber.
The reaction products produced in the combustion chamber are passed between the inner and the outer pipes into the scrubbing chamber and from there via an exhaust into the ambient air.
A wide variety of gases, such as for example, SiH4, PH3, B2H6, TEOS (tetraethoxysilane) from CVD processes, C2F6, CF4, CH3F, Cl2, BCl3 from dry etching processes and from other processes, can be disposed of with great effectiveness by such a waste gas cleaning system. A requirement is that in each instance, the parameters of the waste gas cleaning system be adapted to the type and quantity of the gases or vapors to be cleaned, so as to ensure that combustion or thermal decomposition takes place under excess oxygen.
In such waste gas cleaning systems, the relatively high operating costs and great consumption of media are disadvantageous. Also disadvantageous is that many different media (burnable gases, oxygen) are required and that fairly high installation costs are to be expected. In addition, the use of burnable gases requires special care.
In order to avoid these problems, it has been attempted to make use of low-pressure plasmas for waste gas disposal, such as, for example in
EP-A-0,821 995, the use of a hollow cathode discharge has been proposed. There the disadvantage is that intervention in the vacuum system of the coating or etching system cannot be avoided, so that waste gas disposal may negatively influence the preceding semiconductor process. If this is to be avoided, an additional high-cost vacuum system and possibly a buffer chamber for the temporary storage of process waste gases would have to be provided.
The use of a plasma source that can work under atmospheric pressure would be more favorable. Such plasma sources are operated at an excitation frequency in the microwave range and are capable of generating a non-thermal (cold) plasma.
The use of such a plasma for the disposal of environmentally harmful or toxic compounds has already been proposed.
In this connection, the fact that the plasmas generated often become constricted is to be regarded as disadvantageous. In addition, the plasma volume obtainable is limited by screening effects. As a result, the quantity of disposable waste gas is also limited, so that the quantities of waste gas obtainable in conventional combination systems (burning/thermal decomposition and subsequent scrubbing) cannot be obtained. Therefore particular attention has to be paid to the generation of plasma, in order to avoid the disadvantageous effects listed above. However, even in the most careful design of the plasma source, it is necessary to reduce the pressure in the reaction chamber for ignition of the plasma to some 10 mbar. In addition, the fact that when working under normal pressure the most favorable plasma parameters for conversion of the gases or vapors are not always adjustable proves to be disadvantageous. Additionally, a scrubber is required to remove the harmful substances remaining in the plasma after conversion of the waste gas.
Therefore, the object of the invention is to provide a waste gas cleaning system in which use of a plasma source working even at atmospheric pressure permits effective and low-cost disposal of environmentally harmful or toxic gases or vapors.